1. Field of the Invention
The present invention relates to tuners in which a signal is combined with a local oscillation to form a heterodyne frequency at a given intermediate frequency. The invention also relates to circuits wherein automatic frequency control of the oscillator frequency is provided to avoid drift.
2. Description of the Prior Art
A tuner for converting signals lying over a band of frequencies to a fixed intermediate frequency typically comprises a mixer circuit and an oscillator circuit, both of which are tuned in synchronism and an output circuit for selecting the heterodyne at the desired fixed, intermediate output frequency. In the FM band, in which the present tuner is designed to operate, the frequency is sufficiently high (88-108 megahertz) that high frequency discrete transistors give better performance per unit of cost than general purpose devices formed by currently available integrated circuit processes. At this high frequency, it is recognized that oscillator drift is a problem that has to be dealt with by automatic control. The conventional circuit uses the collector barrier capacity of the oscillator transistor, which has a capacity dependent on the collector potential, or the emitter diffusion capacitance which depends on current. Assuming that the tuner is to be used in an AM-FM receiver, the tuner circuit should be readily switched on and off to suit the mode setting of the receiver. Performance considerations generally lead to more expensive circuitry, and accordingly once a certain performance plateau has been reached, the next objective is to achieve the same or nearly the same performance with simpler, less expensive circuits.